1. Field of the Invention
The present invention relates to a lightwave filter which allows light signals with a plurality of lightwaves to be selected and extracted from a multiplexed light signal, and to a lightwave selective router which makes use of this.
2. Description of the Related Art
There are lightwave filters which allow light signals with a plurality of lightwaves to be selected and extracted from a multiplexed light signal. One example of a conventional lightwave filter of this sort is to be found in the preliminary collection of papers read at the International Conference on Optical Fiber Communication 1996 (Reference Material 1). The lightwave selective filter which is disclosed in this paper is configured by connecting a plurality of reflected lightwave variable gratings rectilinearly. The wavelength of the light signal which each grating reflects basically corresponds one by one to the wavelengths of the multiplexed light signal, and this reflection wavelength is known as the basic reflection wavelength. Since light signals can be allocated to them, each multiplexed wavelength is also referred to as a wavelength channel, irrespective of whether a light signal is actually allocated to it or not.
The reflected lightwave variable gratings are capable of shifting the wavelength of light signals which they reflect from the basic reflection wavelength to other new reflection wavelengths. A wavelength shifted in this manner is known as a shift wavelength.
When selecting a wavelength, a lightwave filter is able, by causing the reflection wavelength of the grating to shift from the basic reflection wavelength to a shift wavelength, to allow light signals of a specific wavelength through, while reflecting those of other wavelengths.
Moreover, in optical communications using multiplexed optical signals it is desirable, when a light signal of a specific wavelength has been isolated and extracted from a multiplexed optical signal, to ensure the effective use of wavelengths by inserting a new optical signal into the vacant wavelength channel. An example of a lightwave selective router which interchanges light signals of specific wavelengths from among multiplexed light signals of this sort is disclosed in the Journal of Lightwave Technology 14: 1320-40, 1996 (Reference Material 2). In the lightwave selective router illustrated in FIG. 17 of Reference Material 2 in particular, a wave-dividing element, whereby the multiplexed light signal is divided by wavelength channel, is connected along with a wave-joining element to each wavelength channel by means of optical fibers. A 2.times.2 optical switch is provided on the light propagation path of each optical fiber, enabling each wavelength channel's light signals to be inserted or isolated by the operation of these 2.times.2 switches.
However, in the case of high-density multiplexed light signals, the wavelength interval between neighboring wavelength channels is short. Thus, use of the lightwave filter described in the above-mentioned paper with, for instance, a high-density multiplex where the wavelength interval is about 1 nm would result in the shift wavelength from the basic reflection wavelength of one grating overlapping with the wavelength of the wavelength channel neighboring this basic reflection wavelength. In such a case, for instance, it becomes difficult to extract light signals of a plurality of neighboring wavelengths by allowing them to pass selectively through a lightwave filter. Otherwise, if a wavelength is to be prevented from overlapping neighboring wavelengths, it is necessary to shift the reflection wavelength into a wavelength band outside the multiplexed band.
For this reason, it has been hoped that a lightwave filter would materialize which would be capable of extracting light signals of a plurality of wavelengths even in the case of high-density multiplexing.
Moreover, the lightwave selective router which is described in the second paper mentioned above makes use of a 2.times.2 optical switch, but it has hitherto been impossible to obtain a 2.times.2 optical switch with a sufficiently high switching speed. What is more, 2.times.2 optical switches in general require a large amount of motive power, it is difficult to reduce crosstalk sufficiently to improve the accuracy of the selected wavelength, and they are dependent on polarization. Thus, the speed of switching wavelengths and the accuracy of the selected wavelength in conventional lightwave selective routers have been restricted by 2.times.2 optical switches. For this reason, it has been hoped that a new lightwave selective router would materialize which would not need to make use of a 2.times.2 optical switch.